For the purpose of suction and delivery control, in currently used compressors, self-acting valves are employed comprising a closing member in the form of a disk or a plate, a seat, a lift stop, and a spring.
Valves represent the most critical units of a compressor. They must meet the following requirements:
have an equivalent area large enough to minimize the energy loss,
be capable of immediately opening in response to a small excess pressure,
provide timely closure at the end of suction and delivery strokes,
ensure a hermetic seal when closed,
have a low volume of dead space, and
possess high strength and resistance to wear.
The higher the rotational speed of the compressor shaft, the average speed of the piston, and the gas density, the more difficult it is to provide a valve to meet all the above requirements. For example, in order to reduce the hydraulic loss, it is necessary to lower the gas velocity in the valve; this, however, can be only achieved at the expense of a larger size and a greater number of valves, which is limited by design capabilities and is incompatible with the goal of reducing the dead space volume.
On the other hand, as the compressor rotational speed is increased, with the mass of the compressor moving parts being kept constant, the spring force must necessarily rise. The spring reinforcement, however, invariably results in additional throttling of gas and in a lower output of the compressor.
For a high rotational speed of the compressor, it would be possible to increase its output and power rating by minimizing the mass of the closing member, such as disk of plate, the penalty of this being loss of strength.
Valves currently in use may be generally classified as disk valves and plate valves. Disk valves, also including ring valves, are not of the straightway type. With alternate loads on the closing member, both the closing member and the valve seat are liable to be damaged with resulting loss of sealing and overflow of the hot compressed gas, which, in turn, leads to carbon or oil scum formation and to consequent failure of the valve.
In addition, during operation, the spring-loaded closing member is rotated by vibrational forces and the spring torque, which results in a nonuniform contact of the closing member and the seat. The manufacture of such valves is most complicated, since it requires high precision finishing of the seat and closing member surfaces as to roughness and planeness. The fabrication problems are further aggravated by the fact that the main operating members of the valve are made of hard-to-machine high-tensile steel.
Nor are plate valves generally of the straightway type. They are provided with a single-path or multi-path seat and a valve plate shaped as a disk, a ring or a rectangular strip. When free, they contact the seat, but, under gas pressure, they are bent along the depression arc in the lift stop means.
In such valves, the seat section is utilized nonuniformly over the length thereof, since the height of the gas flow slot formed by the bending of the plates, somewhat varies. Besides, the maximum lift of the plate is made small from the standpoint of durability, which has a prohibitive effect on the valve output. Furthermore, the gas flow turbulence at the outlet of plate valves is very high, thus increasing the temperature and, consequently, shortening the valve life.
Straightway valves are also known in the prior art. These valves differ from the other types not only in the direction of the gas flow between the parallel plates (cf. straightway valves made by Hoerbiger Ventilwerke Aktiengesellschaft, Austria), but also larger passage sections for the overall dimensions specified. The valve plates clamped at one end impede the free passage of gas to a lesser extent, thus eliminating turbulence problems and resulting in a more economical and efficient valve as well as compressor unit as a whole.
The single-ended clamping of the plate in such valves, however, leads to its vibration in the gas stream and hence to the overflow of gas in operation.
A straightway valve is known comprising a body with sides, parallel end faces, and a projection on one of the end faces, with seat surfaces formed within the body on the sides thereof, flexible closure plates formed by a longitudinally U-shaped clamp with lateral support elements carrying reed-type closing members, said closure plate being attached to the body projection by means of a cotter pin.
Such an arrangement of the valve, however, involves a comparatively large dead space volume in the body projection area, thereby decreasing the output of the valve.
Besides, the cotter pin joint considerably reduces the reliability of the valve, since the gap existing between the cotter pin and the plate and as a result of appreciable vibrations are liable to subject the cotter pin to premature fatigue and consequent fracture.
The cotter-pin attachment of the plate fails to prevent longitudinal displacement of the closure plate relative to the outlet port because of a loose fit of the cotter pin within the opening, thus entailing abrasion both of the seat surface and of the plate and resulting in a shorter life of the valve.
The cotter pin joint makes it impossible to replace the plate indefinitely, for once the cotter pin has been replaced, an increase in the opening size occurs. These shortcomings make the valve difficult to repair.
The arrangement described above fails to provide for a sufficient ease of manufacture as a result of the closure plate being U-shaped in longitudinal section and because of the cotter pin opening in the body projection.